Standard enthalpy change denoted

However, each of the individual reactions involves the formation of a compound from its elements or the decomposition of a compound into those elements. The standard enthalpy change of a reaction that involves the formation of a compound from its elements is referred to as the enthalpy (or heat) of formation of that compound and is denoted by the symbol AH. Thus, for the dehydration of n-propanol. 

The magnitude of any enthalpy change depends on the temperature, pressure, and state (gas, liquid, or solid crystaUine form) of the reactants and products. To compare enthalpies of different reactions, we must define a set of conditions, called a standard suite, at which most enthalpies are tabulated. The standard state of a substance is its pure form at atmospheric pressure (1 atm) and the temperature of interest, which we usually choose to be 298 K (25 °C). The standard enthalpy change of a reaction is defined as the enthalpy change when all reactants and products are in their standard states. We denote a standard enthalpy change as AH°, where the superscript ° indicates standard-state conditions. 

The measured enthalpy change for a reaction has a unique value only if the initial state (reactants) and final state (products) are precisely described. If we define a particular state as standard for the reactants and products, we can then say that the standard enthalpy change is the enthalpy change in a reaction in which the reactants and products are in their standard states. This so-called standard enthalpy of reaction is denoted with a degree symbol, AjH . 

We have learned that if the reactants and products of a reaction are in their standard states, the enthalpy change is a standard enthalpy change, which we can denote as Aj.H°. One of the primary uses of standard enthalpies of formation is in calculating standard enthalpies of reaction. 

The reaction is exothermic and the enthalpy change AH° is therefore negative. The heat of reaction —AH° is positive. The superscript ° denotes a value at standard conditions and the subscript r implies that a chemical reaction is involved. 

The enthalpy and Gibbs free energy changes for a standard reaction are denoted by the... 

The enthalpy that accompanies a change of physical state at standard conditions is called the standard enthalpy of transition and is denoted A trsH°. Enthalpy changes accompanying chemical reactions at standard conditions are in general termed standard enthalpies of reaction and denoted A r H0. Two simple examples are given in Table 1.3. In general, from the first law, the standard enthalpy of a reaction is given by... 

The standard enthalpy of formation of a substance (AfHx) is the enthalpy change involved at temperature T when one mole of the substance is formed in its standard state from its elements in their standard states. The symbol is used to denote standard state. It follows from the first law that the enthalpy change in the reaction... 

Solution An enthalpy increment of 5 kcal mol-1, although modest by chemical standards, is highly significant compared with the ambient RT value of about 0.6 kcal mol-1 near T 300K. If we denote the new binding constant with a prime, and consider only the net enthalpy change A A H° = A Hof - A H° -5 kcal mol-1, we can see from (8.29) that... 

Enthalpy of Formation The enthalpy of formation of a compound at the given temperature may be defined as the enthalpy change when one mole of a compound in its standard state is formed form its elements (in their standard state). It is usually denoted by DH, e.g.,... 

The authors reacted Na2Se03(cr) with a copper sulphate solution in an electrically calibrated calorimeter and measured the enthalpy change of the reaction. The product was CuSe03-2H20(cr) as shown by chemical analysis and X-ray diffraction. Crystalline anhydrous copper selenite was also prepared and the integral enthalpies of dissolution of the two selenites in 8% HMO3 (HNO3, aq 1 40, and denoted sin below) were determined. The data have been used in Table A-45 to calculate standard enthalpies of formation of the copper selenites. 

The heat of reaction, or more accurately, the enthalpy change during a chemical reaction, A, indicates the amount of energy being absorbed or released when a chemical transformation takes place at given operating atmosphere and temperature of 298 K. The standard molar heat of reaction of a chemical reaction (expressed in energy per mole extent) is denoted by It is calculated by... 

Here the R is the gas constant the H in the nomenclature DH° emphasizes that the BDE is the enthalpy change of fhe dissociation processes. The superscript 0 denotes the standard conditions of thermod)mamics. 

The enthalpy and Gibbs free energy changes for a standard reaction are denoted by the symbols A//° and AG°, where the superscript zero is used to signify that a standard reaction is involved. Use of these symbols is restricted to the case where the extent of reaction is Imol for the reaction as written with a specific set of stoichiometric coefficients. The remaining discussion in this chapter refers to this basis. 

The energy that must be supplied as heat at constant pressure per mole of molecules that are vaporized under standard conditions (that is, pure liquid at 1 bar changing to pure vapor at 1 bar) is called the standard enthalpy of vaporization of the liquid and is denoted (Table 1.2). For example, 44 kj of heat is... 

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